How to Understand My Daughter Better?

by A Wondering Mother
(UK)

My daughter is 15 and she never wants to be in the house.
She lives with her foster parents.
She has permission to stay with me once a week, but when she comes down.

She is always really cheeky and slobbers at me.
She says that I do not understand why she is being like this.

How can I understand her?
She was never like this a year ago!
I think she may have some sort of behaviour issues but how do I find out?